The prior art is replete with devices which are generally characterized as paper clips, some of which may also serve as indexing tabs. The primary function of such devices is to provide an adequate clamping action normal to the clamped sheets without damaging the sheets or the clamping device itself. Importantly, the clamping device must be capable of simple and inexpensive fabrication.
Most prior art paper clip devices rely on the resiliency of the device to urge two separable members together. Such devices, by and large, are unsatisfactory for clamping more than an insignificant number of sheets together; those which do effectively clamp large stacks of sheets together are complex and costly to fabricate.
After duly considering this problem I have concluded that paper clips should properly employ a snap action whereby the clip is switchable between two stable positions. The only bi-stable paper clip I have been able to find in the prior art is disclosed in German Pat. No. 80280 (Schwegler, 1894). That paper clip includes a flat sheet and a dimpled top sheet, both made of metal and joined along one edge. In the open position, the dimple is oriented convexly toward the flat sheet so that the free end of the top sheet curves away from the flat sheet. Papers can be inserted between the sheets so that, when the top sheet is urged by finger pressure toward the flat sheet, the dimple snaps outwardly to force the free bent-over and serrated edge of the top sheet to clamp the papers against the flat sheet surface. The clip can be opened by pressing the now outwardly convex dimple inward. While the top sheet of this clp is, in fact, bi-stable, it has several disadvantages. Firstly, the clamping action is achieved only among the bent-over edge of the top sheet which makes a linear contact with the engaged stack. This extremely small contact area is not sufficient for clamping stacks made up of a significant number of sheets. Moreover, if the bent-over edge of the top sheet of the clamp is serrated, as disclosed, it is likely to multilate the top document sheet. In any case, the clamping force of the Schwegler paper clip does not prevent dislodging of the clamped sheets in response to forces exerted on the sheets along their planes. Another disadvantage of the Schwegler clamp resides in the fact that the stresses set up in the top sheet of the clamp during deformation of the dimple result in only a small number of switching operations before the top sheet of the clamp ruptures, particularly if stacks of any significant number of documents are clamped.